Deposition of a metallization layer, typically of aluminum, is a common processing step in the fabrication of very large scale integrated (VLSI) circuits on semiconductor substrates or wafers. Usually, a large number of discreet devices, sometimes referred to as integrated circuit "chips", are formed on a single wafer. Metal layers are typically used as device interconnects which are deposited only after a complex device structure has already been formed on the wafer. Frequently, it is also desired to fill small holes, known as vias, with metallization to provide electrical connection between device layers; and/or to fill narrow grooves in such devices.
Presently, the most common method of depositing thin films of aluminum or other metallization layers is by the well known process of sputtering, a form of physical vapor deposition. In a sputtering system, a plasma of an inert gas at relatively low pressure, typically argon, is created in the vicinity of a target cathode made of the material to be deposited. Ions from the plasma strike the target cathode causing atoms of the target material to be ejected. These atoms travel through the sputtering chamber and are deposited onto the semiconductor substrate. In so-called magnetron sputtering systems a magnetic field is created in the vicinity of the target to confine the electrons and intensify the plasma, thereby increasing the efficiency of the sputter source. In modern commercial sputtering systems substantially all of the atoms released from the target remain neutral i.e. approximately 98% or greater are un-ionized as they travel through the sputter chamber to the substrate. Also, the vast majority of the 2% of ionized target ions which may be formed would usually be confined by the fields along with the electrons and would not reach the substrate.
An ongoing trend in semiconductor device design is towards ever-smaller device geometries, such that the vias and grooves that must be filled with metal are now frequently less than a micron in width. This has presented a problem in connection with sputtering. It is generally understood that atoms ejected from the surface of a sputter target leave at a variety of angles and that, at the vacuum levels typically employed in sputtering systems, the mean-free-path of the ejected metal atoms is small in comparison to the distance between the target and the substrate, so that randomizing collisions occur. Thus, the metal target atoms are incident on the substrate over a wide range of angles, generally conforming to a cosine distribution.
on the other hand, when filling a via or groove of very small width, it is important that it be filled from the bottom up. If there is significant deposition on the side walls of the via or groove before the bottom is filled, then these side layers will block atoms from reaching the bottom with the result that good electrical connection will not be made. It should be apparent that significant side wall deposition will occur in the case where sputtered atoms can arrive at the wafer at angles defined by a cosine distribution.
Accordingly, it has been a goal of manufacturers of sputtering systems to provide means for imparting greater directionality to the ejected target atoms which reach the wafer. Ideally, for filling vias and grooves, sputtered atoms should arrive at an angle which is normal to the plane of the wafer.
A variety of approaches have been tried to impart greater directionality to the sputtered atoms reaching the substrate. One approach is to increase the distance between the sputter source and the substrate. Ignoring, for the moment, the effects of gas scattering, if the distance is relatively large in comparison to the diameters of the source and of the substrate, only those atoms which start out travelling at an angle close to an angle normal to the substrate will reach the substrate. This follows from the geometry of the arrangement. It should be apparent, however, that using this approach any improvement in directionality comes at a cost in system efficiency. While the geometry selects only those atoms with the proper angle of departure from the target, any other atoms ejected from the target are wasted with the consequence of very poor target utilization and a slow deposition rate. Economically, a key factor in modern commercial semiconductor fabrication is the need for increased system "throughput." Thus, an approach which increases deposition time is economically unacceptable.
Another known method of imparting greater directionality to the atoms reaching the wafer surface is to install a collimating filter between the source and the substrate. Such a filter might comprise a network of elongate cell-like structures, each cell having an axis perpendicular to the surface of the substrate. Atoms travelling approximately perpendicular to the substrate surface travel though the cells unimpeded. Atoms travelling at an acute angle are intercepted by a wall of one of the cells and captured. This approach, while providing good directionality, is also inefficient since much of the target material is wasted and builds up on the cell walls. The build up of material can lead to the undesired increase in the number of particulates in the system, making it necessary to replace or clean the collimator frequently. Nonetheless, this approach is an improvement over the approach of the preceding paragraph insofar as it allows for a more compact system.
As noted above, even if directionality is attained using either of the above methods, gas scattering reintroduces randomness in the travel angles of the target atoms. (This is less of a factor with respect to the collimation system because the overall path length is shorter and the mouths of the cells may be placed close to the wafer surface.) It is difficult to reduce gas pressure, and thereby increase the mean-free-path of the ejected atoms, without greatly diminishing the plasma density and the deposition rate. As noted above, slow deposition rates are unacceptable to commercial semiconductor device manufacturers because of the consequent decrease in system throughput.
Another solution which would provide directionality to the metal to be deposited would be some form of ion plating using an ion beam such as used in ion implantation. Ion trajectories can be controlled using known magnetic or electrostatic focussing techniques so that the ions may be directed normal to the surface of the wafer. However, the deposition rate would continue to be a problem if one were to use typical ion implantation beams, because space charge effects in these machines would prevent the use of a beam with sufficient flux to provide an acceptable deposition rate.
Another approach to improving the ability of a sputter source to fill grooves and vias has been to apply an rf bias to the wafer substrate, thereby causing a negative charge to build up in a known manner. This negative charge causes gas ions in the sputter chamber to bombard the wafer imparting a degree of surface mobility to the deposited aluminum atoms causing them to spread out along the surface. While this approach has been useful, it is limited by the fact the energy of the ions striking the substrate must be below a level which will cause damage to the partially fabricated devices present on the substrate.
Accordingly, it is an object of the present invention to provide a physical vapor deposition source for depositing metallization layers onto semiconductor wafers with an improved degree of directionality.
Another object of the present invention is to provide a directional source for depositing metal layers which has an acceptably high deposition rate.